A la carte but with demo versions that allow you to really test the individual amps/cabs.
Marshalls, Mesas, and Peaveys have been done to death sim-wise. I wouldn't be interested in these at all. As a consumer I'd much prefer sims of Sunns, Orange, Laney, or Fortins.
As for the idea of Thermionik, I'd prefer few ampsims (3-5) with the plugin & a possibility to demo/purchase future addons
instead of 20 ampsims at once. Same problem with IRs, there are so many, when you only need a few great ones.
Im still on the hunt for a good amp sim that will work for rock / grunge / pop etc. Hi gain models (metal based sounds) are what i have a ton of already.